The Green Revolution: PLA Production for a Sustainable Tomorrow
In response to the environmental hazards posed by non-biodegradable synthetic plastics, bioplastics have emerged as a compelling solution (Kweku et al., 2018). These conventional plastics found ubiquitously in packaging, textiles, furniture, and medical equipment, persist indefinitely in the environment and significantly contribute to global pollution. The alarming statistics reveal that approximately 400,000 barrels of oil are consumed daily to fuel the production of these plastics (Yaradoddi et al., 2016), and billions of tonnes of plastic waste now burden our ecosystems. Amid this crisis, bioplastics have gained prominence for their ability to biodegrade within 180 days under specific conditions (Shruti & Kutralam-Muniasamy, 2019). They are derived from renewable resources and are free from harmful chemicals, offering a safer and more sustainable. (Bezirhan & Bilgen, 2019; Gadhave et al., 2018)
Poly(lactic acid) (PLA) has emerged as a standout bioplastic in the bioderived monomer category (Song et al., 2011). Poly(lactic acid), derived from lactic acid (LA) (Sin and Tueen, 2020), is a linear aliphatic thermoplastic polyester classified explicitly within the poly-𝛼-hydroxyacid family (Ranakoti et al., 2022). Notably, PLA is 100% biodegradable and economically competitive compared to other biobased and biodegradable plastics. PLA’s production relies on renewable resources like sugarcane, sugar beet, corn, wheat, and potatoes. It finds versatile applications in food service, beverage containers, cups, and bags (Coles, 2013). PLA’s exceptional biodegradability and cost-effectiveness have positioned it as a crucial player in mitigating the environmental impact of conventional plastics.
The urgent need for sustainable alternatives to synthetic plastics has driven the adoption of bioplastics, with PLA leading the charge. PLA’s environmental compatibility, versatility, and cost-effectiveness underscores its vital role in addressing the global plastic pollution crisis. As consumer demand for eco-friendly materials grows, PLA’s prominence in various industries is expected to increase, furthering the transition toward a more sustainable and environmentally conscious future.
Now that we’ve established PLA as a solution to plastic waste, is there a significant market demand for poly(lactic acid)?
The Skyrocketing Demand for Bioplastics
Poly(lactic acid) is gaining substantial ground in the global market. While bioplastics currently represent less than one percent of the massive 390 million tonnes of plastic produced annually, recent trends suggest a promising trajectory. Global plastic production stagnated in 2020 due to the Covid-19 pandemic and has rebounded since 2021. This resurgence is propelled by increasing demand and the emergence of more sophisticated applications and products. Notably, the bioplastics market is anticipated to experience an astounding compound annual growth rate (CAGR) of 18.8% from 2023 to 2030, projected to reach a staggering USD 444,772.3 million by 2030. (European Bioplastics & Nova-Institute, 2022; Fortune Business Insights, 2023)
Market data from European Bioplastics in partnership with the nova-Institute unveils remarkable growth in bioplastics production capacities. From approximately 2.2 million tonnes in 2022, global bioplastics production is poised to skyrocket to about 6.3 million tonnes by 2027. This surge is attributed to the diversification and expansion of bioplastics, including PLA, PHA (polyhydroxyalkanoates), PAs (polyamides), and Polypropylene (PP), propelled by advanced polymer development. Furthermore, the global market value of PLA is projected to soar to some 2.8 billion U.S. dollars by 2030, marking a substantial transformation from its estimated value of US$ 1.0 Billion in 2022. The PLA market is expected to continue its remarkable growth trajectory, with forecasts predicting it to reach US$ 3.1 Billion by 2032, growing at a CAGR of 11.9%. (Fortune Business Insights, 2023)
Biodegradable plastics, including PLA, PHA, starch blends, and others, already account for more than 51 percent (over 1.1 million tonnes) of global bioplastics production capacities. The trajectory is upward, with the production of biodegradable plastics projected to exceed 3.5 million tonnes by 2027. This growth is fueled by the remarkable development of polymers like poly(lactic acid)s (PLAs) and polyhydroxyalkanoates (PHAs).
In this landscape, PLA emerges as a standout player. In 2022, PLA comprised 20.7% of the total bioplastics production, and it’s expected to surge further, reaching a substantial 37.9% share in 2027. The rise of PLA underscores its versatility and eco-friendly attributes, making it a significant contributor to the ever-expanding world of bioplastics. As the demand for sustainable materials intensifies, PLA’s prominence is set to reshape the plastics industry, ushering in a greener and more responsible era.
Having observed the surging demand for PLA, what are its diverse applications and market uses?
Market and Applications of PLA
PLA’s versatility is exemplified by its diverse applications across various industries (Sin & Tueen, 2019). Corbion, formerly known as Purac, is the world’s largest lactic acid producer. Their lactic acid plant in Thailand, with an annual output of 120,000 MT in 2016, has ambitious plans to increase its capacity to 205,000 MT annually in the future. Corbion’s global operations span the Netherlands, Spain, Brazil, and the United States, supplying over 60% of the world’s lactic acid. Additionally, Corbion-Purac is pivotal in manufacturing PLA and PLA copolymers for biomedical applications, including sutures, pins, screws, and tissue scaffolding materials.
Moreover, companies like CL Chemical Fibers utilize PLA for spun-bound fabrics, catering to medical applications, shopping bags, and landscape textiles. Dyne-A-Pak employs PLA in creating foam meat trays for Dyne-A-Pak Nature Trays, while Bodin in France utilizes PLA for foam trays used in packaging meat, fish, and cheese. CDS-SRL produces cutlery, Cargo Cosmetics utilizes PLA for cosmetics casings, and DS Technical Nonwoven manufactures exhibition-grade carpeting (Ecopunch carpet) from PLA. An array of companies, including Sant’Anna, Swangold, Cool Change, Good Water, Primo Water, Nature Organics, Naturally Iowa, Priori, and Fritto-Lay, incorporate PLA into various products, from bottles for beverages and cosmetics to packaging materials, further propelling PLA’s growth in the global market.
The Manufacturing of Poly(lactic acid)
The production of PLA is a multifaceted process driven by innovations in two primary routes: the Direct Polycondensation (DP) route and the Ring-Opening Polymerization (ROP) route (Sin & Tueen, 2019). While DP offers simplicity in PLA production, ROP can yield a low-molecular-weight, brittle form of PLA. It’s essential to distinguish between “poly(lactic acid)” and “poly(lactide)”, where the former is produced through DP and the latter via ROP. The term “poly(lactic acid)” is often used interchangeably, though a scientific distinction exists. There is a surge in the adoption of PLA production methods due to their eco-friendliness.
1. Lactic Acid Production
The foundation of PLA production lies in Lactic Acid (LA) synthesis. LA is obtained through the fermentation of sugars, where microorganisms like Lactobacillus species play a pivotal role. This process converts sugars from renewable sources, such as corn starch or sugarcane, into LA. The fermentation process typically lasts 3–6 days under mildly acidic conditions and specific temperature ranges. Various nutrients and substrates are employed to ensure the optimal reactivity of LA bacteria. Most of the global LA production relies on the fermentation process, primarily carried out by Lactobacillus bacteria.
— Example (Ranakoti et al., 2022)
One variation in the manufacturing process utilizes ammonia in direct starch fermentation. The fermentation process commences with the application of starch in an ammonia-rich environment within a specialized reactor. This reactor facilitates the production of ammonium lactate while concurrently separating acetic acid, carbon dioxide, and alcohol as byproducts.
2. Lactic Acid Purification
The LA obtained from fermentation undergoes rigorous purification processes before it can be used for PLA production. Techniques such as electrodialysis, reverse osmosis, liquid extraction, ion-exchange acidification, and others are employed. The high optical purity of L-lactic acid (>99%) is essential for specific applications like food and pharmaceuticals. Avoiding extreme conditions is crucial to prevent the conversion of D-lactic acid and L-lactic acid into each other, forming a racemic mixture. High optical purity of L-lactic acid (>99%) is required for food and pharmaceutical applications to meet stringent requirements for oral intake. Selectivity of a single optical LA is preferable for quality control since different optical LAs can affect PLA properties, including melting point, mechanical strength, and degradability.
— Example (Ranakoti et al., 2022)
As one example, electrodialysis plays a pivotal role in the industrial manufacturing process. After following the previously described steps, ammonium lactate is directed to water-splitting chambers, where electrodialysis is applied to effectively separate ammonia from the ammonium lactate. This separation process harnesses the electrochemical potential of cation and anion interactions, ultimately leading to the production of lactic acid.
3. Lactide Production
Lactide, an intermediate substance in PLA production via the ROP method, is produced by further processing concentrated LA. The crude LA, consisting of approximately 15% LA and 85% water, is initially concentrated through evaporation. The concentrated LA then undergoes condensation polymerization in a prepolymer reactor to form PLA with the desired molecular weight. Subsequently, the oligomer LA is fed into a lactide reactor, where various catalysts can be used to facilitate the reaction. Many suitable catalysts, including metal oxides, metal halides, metal dusts, and organic metal compounds derived from carboxylic acids, can be used.
— Example (Ranakoti et al., 2022)
The prepolymer exiting the oligomerization reactor proceeds into a depolymerization reactor, where each monomer within the free polymer is disassembled into its constituent sub-units and is then shaped into lactide rings, which are essential for the subsequent ring-opening process. This results in the formation of three distinct stereo isomers of lactide, namely L-lactide, D-lactide, and meso lactide. These three stereo isomers are conveyed to a purification column, where heat is utilized to facilitate the separation of meso lactide, which exists in liquid form, from L- and D-lactides.
4. Poly(lactic acid) Production (Polymerization)
Large-scale PLA production predominantly relies on the Ring-Opening Polymerization (ROP) of lactide, resulting in high-molecular-weight PLA. Various catalyst systems, including metal catalysts, are employed to achieve fast and high-yield lactide polymerization. Stannous octoate (tin octoate) is a commonly used catalyst, yielding high molecular weight PLA (>250,000). The catalytic ROP reaction also allows for copolymerization of lactide with other monomers, broadening PLA’s application range. Transition metals like aluminum, zinc, tin, and lanthanides are commonly used as catalysts, ensuring the safety of PLA in food packaging and biomedical applications while achieving high molecular weights. The highly effective catalyst is applied at a low level (<10 ppm) to ensure the safety of PLA when used in food packaging and biomedical applications.
— Example (Ranakoti et al., 2022)
Following the separation of meso lactide, L- and D-lactides are introduced into a polymerization reactor, where ring-opening takes place. A metal catalyst is crucial for this process, with options including zinc, titanium, and tin. Tin, particularly in the form of stannous octoate (Sn(Oct)₂), is a widely recognized catalyst for lactide polymerization. When combined with alcohol, stannous octoate forms a complex molecule that enables the attachment of ester oxygen to lactide.
This results in the opening of the oxygen’s double bond, creating a chain of poly(lactic acid). Stannous octoate initiates the opening of numerous lactide rings, leading to the formation of a long PLA chain. Alternatively, carboxylic acid can be used as a catalyst instead of alcohol, although it is less common due to lower PLA production yields. Although PLA is produced in the polymerization reactor, Sn(Oct)₂ remains attached to it and is removed in the extruder and pelletizer chamber. Sodium carbonate serves as a catalyst, reacting with Sn(Oct)₂ and separating through a string. The resulting pure liquid PLA is solidified using a dryer and molded into the desired shapes. This flexible process can simultaneously extract D, L, and meso compounds, and changing the catalyst can achieve a high PLA yield.
5. Poly(lactic acid) Post-Processing
PLA can undergo various processing techniques:
a. Extrusion
The poly(lactic acid) (PLA) extrusion process involves three key stages: continuous melting of PLA, conveying, and PLA extrusion through a die. PLA material can be melted and compounded using an extruder to create PLA compounds, which can then be further processed into final products using methods like blow molding, injection molding, thermoforming, and more.
b. Blow Molding
The substitution of non-biodegradable polymer beverage bottles (e.g., polypropylene, polyethylene) with biodegradable alternatives like PLA has been adopted by some food and beverage industries, driven by increasing environmental awareness. However, the use of PLA bottles in the beverage industry is primarily limited to beverages that are not sensitive to oxygen, such as pasteurized milk and still-water beverages.
c. Injection Molding
Injection molding is one of the most widely employed processing techniques for transforming polymers, including PLA, into thermoplastic end products with intricate shapes and demanding dimensional precision.
d. Thermoforming
Thermoforming involves pressing preheated flexible plastic into the desired shape using vacuum air pressure. This method is frequently used for manufacturing PLA packaging containers with straightforward features, such as disposable cups, food trays, lids, and blister packaging.
All in All…
In response to the pressing environmental issues posed by non-biodegradable plastics, bioplastics, with PLA as a leading example, have emerged as a strong eco-friendly solution. The widespread use of traditional plastics, particularly in packaging, textiles, and various industries, has caused persistent environmental problems and heavy reliance on finite oil resources. Bioplastics like PLA offer an attractive alternative as they biodegrade within 180 days, providing sustainability from renewable sources while avoiding harmful chemicals. PLA’s exceptional characteristics, including 100% biodegradability and cost-effectiveness, make it a versatile choice for various applications. Despite currently representing less than one percent of the 390 million tonnes of plastic produced annually, PLA’s potential for growth aligns with the promising trajectory of bioplastics. This highlights the crucial role PLA and bioplastics play in addressing the global plastic pollution crisis and guiding us toward a sustainable future.
References
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